The aim of this research was to compare the protective effects of mesna, hyperbaric oxygenation (HBO), and their combination in cyclophosphamide-induced hemorrhagic cystitis in guinea pigs. Following one dose of i.p. 21.5 mg./kg. mesna administration 20 minutes before i.p. 68.1 mg./kg. cyclophosphamide, 3 additional doses of mesna were given every three hours. A total of 8 HBO exposures, 5 of which were applied prophylactically before cyclophosphamide, were performed at 2.8 ATA for 90 minutes 2 times a day. Although mesna or HBO provided significant protection for cyclophosphamide-cystitis in animal bladders, there was also significant damage compared with controls. The combination of mesna and HBO, which act through independent mechanisms, resulted in complete protection, since mean histological scores and hematuria levels in this group were not different from controls (p >0.05). Therefore, this combination may be a useful tool in the prophylaxis and treatment of cyclophosphamide-induced hemorrhagic cystitis.